The objective of this study is to test whether the combination of two effective anti-leukemic drugs - Cyclophosphamide and Adriamycin- administered early in complete remission will increase the eradication of leukemia cells and thus the duration and frequency of initial continuous complete remission in patients receiving 6-Mercaptopurine and Methotrexate continuation chemotherapy. Approximately 60 or more children with acute lymphoblastic leukemia will be admitted to the study. Initial therapy for all patients will consist of Prednisone, Vincristine and Asparaginase. Those in complete remission will receive preventive central nervous system therapy consisting of 2400 Rads cobalt 60 cranial radiotherapy and concommitant intrathecally administered Methotrexate. Those in remission will be randomly divided into two groups to receive continuation chemotherapy. The first group will receive weekly Methotrexate and daily 6-Mercaptopurine. The second group will receive one month of Cyclophosphamide given weekly and Adriamycin given every two weeks, thereafter receiving Mtx. and 6-MP as in the first group. The two groups of patients will be carefully followed and compared for significant differences in remission duration, toxic effects and relapse rates.